The use of inorganic whiskers and fibers to reinforce glasses, glass-ceramics, sintered ceramics, plastics, and metals has long been practiced. Commonly, the term whiskers has been applied to elongated, single-crystal fibers. In general, whiskers have been described as having a thickness less than about 100 microns with a length-to-diameter ratio of at least 100.
Whiskers have found extensive use as reinforcing agents in various matrices because of their inherent shape, high modulus of elasticity, and high tensile strength. To illustrate, when dispersed in a crystalline matrix, whiskers will occupy sites along the grain boundaries of the crystals, and may significantly improve the creep resistance of the material. This may be due, for example, to an increase in the length of shear required and/or the added complexity of shear required to yield apparent creep.
Also, the high elastic modulus and tensile strength of many different whiskers enables them to produce composite products demonstrating superior strength-to-weight and stiffness-to-weight properties. For example, whiskers prepared from very stiff, low density covalent compounds such as carbides, nitrides, and oxides can exhibit elastic moduli higher than most metals, and are often many times stronger than steel when considered in proportion to their weight.
In contrast to whiskers, fibers are generally deemed to be multicrystalline or amorphous. Extensive study to understand the basic means underlying the strengthening improvement to composite bodies imparted by fibers has indicated the mechanism to be that of load transfer by the matrix to the fibers through shear. This load transfer shifts stress to the relatively long, high modulus fibers, and the fibers may additionally act to impede crack propagation in the matrix.
The basic strengthening mechanism is believed to be the same in whisker-containing composites, but the amount of load transferred by the matrix to the whiskers is dependent upon the length and aspect ratio of the whisker. Hence, shorter whiskers may not be loaded to the breaking stress and, consequently, full advantage cannot be taken to their reinforcing capabilities.
In addition to the length and aspect ratio of the whisker, orientation of the whisker with respect to the applied stress and the stress concentrations at the ends of the whisker result in lower strength than would be possible with fibers. Accordingly, whisker reinforced composites will typically manifest less desirable mechanical properties than unidirectionally-oriented, continuous fiber composites fabricated from like constituents (when properties are measured along the fiber axis). Whisker-containing composites possess an advantage, however, over the continuous fiber-containing composites in that they are nearly macroscopically isotropic.
SiC fibers and whiskers have been demonstrated as reinforcing agents in numerous metal and non-metal matrices. For example, U.S. Pat. No. 4,324,843 records the formation of SiC fiber reinforced glass-ceramic composite bodies wherein the glass-ceramic matrix is selected from the composition systems of aluminosilicate, lithium aluminosilicate, magnesium aluminosilicate, and combinations thereof. U.S. Pat. No. 4,464,475 discloses the production of SiC fiber reinforced glass-ceramic composite bodies wherein barium osumilite constitutes the predominant crystal phase. U.S. Pat. No. 4,464,192 describes the preparation of SiC whisker reinforced glass and glass-ceramic composite bodies wherein the glass-ceramic matrix is selected from the group of lithium aluminosilicate, magnesium aluminosilicate, aluminosilicate, and combinations thereof.
The above matrices are asserted to be suitable for use temperatures up to about 1300.degree. C. Above that temperature range those compositions are not refractory enough to provide a viscosity sufficiently high to transfer load to reinforcing fibers and whiskers. Consequently, the matrix deforms excessively and the composite suffers loss of load-bearing ability.
In the field of fiber reinforced glass composites, U.S. Pat. No. 4,464,192 discloses the preparation of reinforced composite articles consisting of whiskers or chopped fibers embedded in a glass matrix. The patent describes in some detail the production, through injection molding, of composite articles consisting of chopped fibers (about 0.75" in length with an average diameter of .+-.5-50 microns) of alumina, graphite, silicon carbide, and/or silicon nitride dispersed within a matrix of a high silica glass, a borosilicate glass, or an aluminosilicate glass. U.S. Pat. No. 4,314,852 discloses the fabrication of reinforced composite articles consisting of continuous SiC fibers embedded in a glass matrix, the glass again being selected from the group of high silica glass, borosilicate glass, and aluminosilicate glass.
The mechanisms of toughening in wholly ceramic matrices, i.e. ceramic matrices without substantial glassy phases, have been reviewed by R. W. Rice in "Mechanisms of Toughening in Ceramic Composites", Ceram. Eng. Sci. Proc., 2(7-8) 661-701 (1981). Major strengthening mechanisms for fibers in these ceramics include load transfer, prestressing, crack impediment, crack deflection, and fiber pullout. Also noted, however, is the fact that second phases incorporated in composites for purposes of reinforcement provide many potential sources and preferred paths for localized stresses and crack growth. Thus some composites may have significantly lower compressive strengths than the pure ceramic matrix itself, or may suffer damage under compressive loading which leads to reductions in tensile strength.
Chemical compatibility between the ceramic matrix and the reinforcing phases is of course a fundamental requirement of any composite ceramic system. U.S. Pat. No. 4,485,179 discloses that silicon carbide fibers, in particular, exhibit high reactivity toward certain glass-ceramic matrix materials. That patent describes a chemical modification of the matrix phase which was used to moderate this activity. As this patent suggests, compatibility is required not only under the conditions of use, but also under the conditions encountered in the course of composite fabrication. For example, silicon carbide has been shown to promote foaming in certain ceramic batches for cordierite products, as shown in U.S. Pat. No. 4,297,140.
Cordierite is a crystalline magnesium aluminum metasilicate material (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) known to exhibit a low coefficient of thermal expansion over a rather wide temperature range. Major proportions of this phase in a ceramic body therefore impart excellent thermal shock resistance to the body.
By virtue of this excellent thermal shock resistance and refractoriness, extruded monolithic ceramic honeycomb structures comprising cordierite or substituted cordierite as the principal crystalline phase have found widespread use as catalyst supports and filters in the treatment of combustion exhaust gases produced by motor vehicles and woodstoves. U.S. Pat. No. 3,885,977 describes the manufacture of such bodies from extrudable batch mixtures of clay, talc and alumina, these components reacting to form cordierite as the extruded body is fired after forming.
While cordierite products such as described in this patent have exhibited adequate strength and thermal shock resistance for many applications, certain applications such as use in motor vehicles involve repeated and extensive physical and thermal shocks. Thus careful packaging is required to minimize the incidence of product breakage. For these applications, particularly, improvements in strength and/or thermal shock resistance in the monolithic cordierite structure would be beneficial.
Accordingly, it is a principal object of the present invention to provide a reinforced cordierite ceramic body offering improved strength and/or thermal shock resistance.
It is a further object of the invention to provide a method for providing reinforced cordierite ceramics from clay-containing batch materials.
Other objects of the invention will become apparent from the following description thereof.